Plasma processing equipment has been developed to generate and confine a low pressure radio frequency plasma discharge for the processing of substrates, such as wafers used in integrated circuit fabrication. Processing may include a number of operations, such as reactive ion etching, plasma enhanced chemical vapor deposition, and sputtering. A magnetron uses magnetic confinement of electrons to increase the density of the plasma adjacent to a substrate surface. The processing rate in a magnetron is thereby increased by the higher overall plasma density so that acceptable levels of production may be achieved--an important concern especially for single-wafer-at-a-time processing.
The magnetron typically includes a magnetic field B generated parallel to the surface of the substrate and an electric field E generated perpendicular to the substrate by a cathode upon which the substrate is typically positioned. Electrons would rotate in closed circles around magnetic field lines in the absence of an electric field; however, since the electric field generated by the cathode tends to decelerate electrons as they approach the cathode, the final paths of the electrons are cycloidal in the E.times.B direction. Thus, an electron storage ring is formed around the cathode of the magnetron thereby producing a higher plasma density.
Magnetron plasma processing technology has developed so called "wings" to electrostatically confine plasma to the region adjacent the substrate surface. Wings are described, for example, in U.S. Pat. No. 4,132,613 to Penfold et al. A pair of fixed parallel wings are positioned to extend perpendicular to the magnetic field and define a region therebetween which permits the electrons influenced by the electric and magnetic fields to circulate in a closed path in the electron storage ring around a cylindrical cathode. Since the wings represent a high impedance to the plasma, only a small electron current flows to the wings. Thus, the plasma is effectively confined in the direction parallel to the magnetic field B.
Despite techniques for increasing the overall density of plasma, such as magnetic or electrostatic confinement, a magnetron may not have a uniform plasma density over the entire substrate surface to be processed. The non-uniform plasma density produces a non-uniform processing rate over the surface of the substrate. As integrated circuit manufacturing technology moves towards larger substrates and finer resolutions, this non-uniformity becomes even more critical.
Magnetrons have also been developed with remote plasma sources to increase the overall density of a plasma without increasing the energy of the ionic flux impinging the substrate. Unwanted energy causes heating, substrate ion bombardment damage, radiation damage, contamination, and loss of selectivity in etching. An example of a magnetron design having a remote plasma source is the "split cathode" magnetron described in U.S. Pat. No. 4,738,761 to Bobbio et al. and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.
As shown in FIG. 1, the split cathode magnetron 10 of the prior art utilizes an upper electrode 11 and a bottom electrode 12 with an insulator 13 positioned therebetween. Each electrode 11,12 has a controllable radio frequency power input rf1,rf2. The electrodes 11,12 are disposed within an evacuable chamber 14 for containing a reactant gas therein. The electric field E is generated in a direction into the substrate surface 15 by the action of the plasma, and the magnetic field B is generated parallel to the substrate surface by an external pair of magnets (not shown), thereby causing an electron drift velocity V.sub.D in the E.times.B direction. Fixed parallel wings 16a, 16b are provided on opposing sides of the substrate to electrostatically confine the plasma.
The split cathode magnetron 10 may be used with either the upper 11 or lower electrode 12 having a higher relative power. The upper electrode 11 may be higher powered when, for example, etching a material requiring a higher energy, such as a polymer. The lower electrode 12 may be higher powered when a material, such as polycrystalline silicon (polysilicon), requiring a lower energy is etched. Unfortunately, either of these relative powering configurations will result in a non-uniform processing rate over the surface of the substrate 15 to be processed. The processing rate of a substrate 15 on the upper electrode 11 will either build in the direction of the electron drift V.sub.D (top electrode higher powered) or the rate will decay in the direction of the electron drift V.sub.D (bottom electrode 12 higher powered). Other magnetrons may also experience a similar decay or buildup of the processing rate over the surface of the substrate. Moreover, increasing the overall plasma density in the region adjacent the substrate does not improve this non-uniformity of the processing rate over the surface of the substrate.
In addition to uniform processing of substrates, certain semiconductor processing applications may require that different areas of a substrate be processed at different rates. The fixed parallel wing electrostatic confinement of the prior art is unsuitable for such selective processing.